Electric and hybrid vehicles have a brake system configured for recuperative braking including an electric motor operated as a generator during recuperative braking. Following an intermediate storage, the electrical energy recovered during recuperative braking may be used for accelerating the vehicle. This makes it possible to reduce power loss, which a conventional vehicle incurs when braking frequently in travel, energy consumption and pollutant emission of the electric or hybrid vehicle.
However, operating the electric motor, for example the electric drive motor, in generator mode typically requires a certain minimum speed of the vehicle. A recuperative brake system is thus frequently unable to exert a regenerative braking torque on the wheels of the vehicle for as long as it takes for the moving vehicle to come a standstill. For this reason, a hybrid vehicle often has, in addition to the recuperatively operated electric motor, also a hydraulic brake system, which makes it possible, at least in a low speed range, to compensate for the lack of braking action of the recuperative brake. In this case it is possible to apply the entire braking torque via the hydraulic brake system even when the electrical energy store is full, which is when the recuperative brake usually exerts no braking torque on the wheels.
On the other hand, in some situations, it is desirable to exert the lowest possible hydraulic braking force on the wheels in order to achieve a high degree of recuperation. For example, following shifting operations, the decoupled generator is often activated as a recuperative brake in order to ensure reliable charging of the intermediate store and high energy savings.
Generally, a driver may prefer a total braking torque of his vehicle, which corresponds to his actuation of a brake input element such as his brake pedal actuation, for example, irrespective of an activation or deactivation of the recuperative brake. For this reason, some electric and hybrid vehicles have an automatic system, which is supposed to adapt the braking torque of the hydraulic brake system to the current braking torque of the recuperative brake in such a way that a desired total braking torque is maintained. The driver thus does not have to take on the task of the delay regulator himself, by adapting the braking torque of the hydraulic brake systems via an appropriate actuation of the brake input element to the current braking torque of the recuperative brake. Examples for such an automatic system are brake-by-wire brake systems, in particular EHB systems. Due to their complex electronics, mechanics and hydraulics, however, brake-by-wire brake systems are relatively expensive.
As an alternative to brake-by-wire brake systems, DE 10 2008 002 345 A1 discusses a brake system, which includes a first brake circuit decoupled from a master brake cylinder and connected to a brake medium reservoir. This first brake circuit has a wheel axle assigned to it, onto which a recuperative braking torque of an electric motor operated as a generator may be applied. Two additional brake circuits are coupled to the master brake cylinder in such a way that the driver is able to brake into them directly and thus exert a direct hydraulic braking torque onto the wheels associated with the two additional brake circuits.